Quenching is categorized into two main types: overall quenching and surface quenching. In surface quenching, induction heating is commonly used due to its efficiency and speed. Induction heating involves rapid temperature rise, and several factors such as the heating rate, current penetration depth, material type of the workpiece, cooling medium used for quenching, and the desired hardening depth significantly influence the phase transformation temperature, kinetics, and final microstructure.
In industrial applications, the composition of the workpiece material plays a crucial role in determining key parameters like the frequency and power of the induction heating system, the moving speed of the sensor, and the gap between the sensor and the workpiece. These settings help control the heating speed and current penetration depth. Additionally, the cooling capacity of the quenching medium and the spray device are adjusted to regulate the depth of the hardened layer, ensuring it meets the design specifications.
**1. Technical Requirements**
The workpiece in question is a large-diameter eccentric gear, consisting of a ring gear and an eccentric body, as shown in Figure 1. The ring gear is made from 45 steel and undergoes surface quenching and tempering. The surface hardness must reach 48HRC, with a hardened layer depth of 3–5 mm to ensure sufficient contact strength, root bending strength, and fatigue resistance during gear meshing.
**2. Induction Quenching Process Parameters**
(1) **Heating Temperature**: The original structure of the ring gear is already tempered and quenched. Due to the high-speed nature of medium-frequency induction heating, the temperature can be slightly increased. For 45 steel, the heating temperature is typically maintained at 850–880°C.
(2) **Frequency and Penetration Depth**: The penetration depth of the current depends on the frequency, following the formula:
$$
d = \frac{K}{\sqrt{f}}
$$
where $ d $ is the penetration depth, $ f $ is the frequency, and $ K $ is a constant depending on the material. Table 1 provides typical values based on production experience. Lower frequencies result in deeper penetration, and for 45 steel, the frequency is usually kept below 8000 Hz to meet hardness requirements.
(3) **Quenching Cooling Medium**: For 45 steel and similar simple shapes, tap water is often used as the cooling medium. The temperature should be maintained between 18–50°C, and the continuous quenching method is applied. The spray device sprays the workpiece at a 45° angle to ensure even cooling.
**3. Process Test Results and Analysis**
By adjusting electrical parameters to meet process requirements, six test samples were quenched. The tooth count varied from 57 to 62, and the test parameters are listed in Table 2. Each sample was tempered and tested for physical and chemical properties, with results summarized in Table 3.
From the data:
- Comparing test numbers 57 and 58, as well as 60 and 62, it was found that lower cooling medium temperatures (20–50°C) increase the hardened layer depth. As the inductor moves faster, this effect becomes more pronounced.
- When the output power increased by 13.5 kW (from 59 to 60), the hardened layer depth increased by approximately 0.6 mm. Higher power leads to stronger eddy currents, resulting in faster heating and higher surface temperatures. However, excessive power may cause surface cracking.
- Increasing the inductor's moving speed by 50 mm/min (from 59 to 61) reduced the hardened layer depth by about 1 mm. This highlights the importance of balancing speed with penetration depth to achieve the desired hardening.
**4. Conclusion**
Factors such as power, frequency, inductor movement speed, and cooling medium temperature all influence the hardened layer depth. Through extensive testing and analysis, optimal parameters were determined for 45 steel gears. A moving speed of 120–150 mm/min, power of 39–45 kW, frequency of 4200 Hz, and a cooling medium temperature of 20–40°C were found to produce a hardened layer of 3–5 mm, meeting the required mechanical properties without cracks.
Author: Li Dan, Xue Wei, Jinan Second Machine Tool Group Co., Ltd.
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